ORIGIN OF IGNEOUS ROCKS. 215 



Amygdaloid. — Still another structure, very common in lavas and 

 traps, is the amygdaloidal. The rock called amygdaloid (Fig. 190) 

 greatly resembles volcanic conglomerate, 

 being apparently composed of almond- 

 shaped pebbles in an igneous paste, but 

 is formed in a wholly different way. 

 Outpoured traps, and especially lava- 

 streams, are very often vesicular — i. e., 

 filled with vapor-blebs, usually of a flat- 

 tened, ellipsoidal form. In the course of 

 time these cavities are filled with silica, 

 carbonate of lime, or some other mate- 

 rial, by infiltrated water holding these 

 matters in solution. Sometimes the fill- 

 ing has taken place very slowly by sue- FlG< ^^Imygdaioid. 

 cessive additions of different-colored ma- 

 terial. Thus are formed the beautiful agate pebbles, or more properly 

 amygdules, so common in trap. The most common filling is silica, 

 because water percolating through igneous rocks is always alkaline, 

 and holds silica in solution. 



Some Important General Questions connected with Igneous 



Rocks. 



1. Origin of Igneous Rocks. 



There are many reasons for thinking that igneous rocks are not 

 erupted portions of an original fused magma, but are usually the result 

 of refusion of stratified rocks. This question has been already touched 

 in treating of volcanoes (p. 100), but we are now in condition to take it 

 up more fully. 



If the earth cooled from a primal incandescent, fused condition, it is 

 evident that there would be substantial homogeneity in any given layer, 

 at least in any given locality. Erupted matters, therefore, although 

 they might indeed slowly change in composition in the course of geo- 

 logical ages, as deeper and deeper layers were successively reached by the 

 gradual thickening of the earth's crust, yet, in the same locality, and 

 erupted about the same time, they ought to have the same composition. 

 But we find, on the contrary, lavas of the greatest differences — e. g., 

 rhyolite and basalt, erupted in the same region, and nearly at the same 

 time. They can not, therefore, be portions of the same original magma. 



Now, in the primal solidification of the earth from fusion, the first 

 crust was doubtless a homogeneous igneous rock, somewhat similar in 

 composition to diorite or syenite. The effect of aqueous agencies on 

 this original homogeneous material, by disintegration, transportation, 



